ath5k: Skip tx power setting on AR5210 for now

 * Don't return -EINVAL when trying to set tx power
 on RF5110 because AR5210 reset will fail. We need to
 add support for RF5110 and AR5210 eeprom in the future
 but for now just skip it.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>